1. Field of the Invention
The present invention relates to processes for producing a printed circuit board. The present invention further relates to resin compositions and adhesive films which are advantageously used in such a process.
2. Discussion of the Background
In recent years, for a printed circuit board used in electronic appliances, communications apparatus and the like, higher processing speeds and higher wiring densities have been strongly required. Accordingly, as a process for producing a multilayer printed circuit board, the production technique of a build-up system in which an organic insulation layer is alternately stacked on a conductor layer of a circuit substrate has attracted much interest. As an insulation resin currently used in the build-up system, aromatic epoxy resins in combination with curing agents having active hydrogen (for example, a phenolic curing agent, an amine-type curing agent, and a carboxylic acid-type curing agent) have been mainly used. The cured product obtained using these curing agents has well-balanced properties, but suffers from the defects that a hydroxyl group having a high polarity is generated by the reaction of an epoxy group and active hydrogen which decreases the moisture resistance and electrical properties such as the dielectric constant and the dielectric dissipation factor. Especially for a multilayer printed circuit board which is to be used in the high-frequency region, an insulation material having a low dielectric dissipation factor has been required. However, with an ordinary insulation material which contains an epoxy resin as a main component, the value of the dielectric dissipation factor (1 GHz, 23° C.) has been limited to from approximately 0.03 to 0.02.
Meanwhile, it has been long known that a cyanate compound having a thermosetting cyanato group gives a cured product with excellent dielectric properties, and an example of applying a thermosetting resin composition containing a cyanate compound to an insulation layer of a circuit substrate has been also known (see, for example, WO 03/099952). However, when an insulation layer is formed on a circuit substrate with a thermosetting resin composition which contains a cyanate compound, a concave-convex gap on the surface of the insulation layer ascribable to concave and convex surface of a circuit pattern has been increased, making it hard to form a fine circuit (see, FIG. 1). Accordingly, a thermosetting resin composition which contains a cyanate compound and which affords an insulation layer which exhibits an excellent flatness on its surface has been in demand.